There is very strong need to test mid-frequency power spectrum density (PSD) for optical surface used in high power laser system, aerospace high resolution camera, astronomy optical system, and X-ray optics, etc. To improve measurement accuracy and remove the error, the calibration of frequency response function for a phase-shifted interferometer must be done before PSD is tested by this interferometer. In this paper, the intensity function contributed by three plane-waves interference is given at first. On basis of this function, the sinusoidal phase grating with variable frequency formed by two plane-waves interference with appropriate ratio of two amplitudes is discussed and is used to calibrate the frequency response function of an interferometer. A test example is given and some error sources are pointed out and studied.